Priority settings

This allows you to change the task priorities of the main Opus 5 process and
of the lister processes.  You can set the priority independently for when
the process is busy and for the rest of the time.

You might need to use this, for instance, if you find a program you use at
the same time as Opus 5 is at a higher priority and is hogging the CPU,
preventing Opus 5 from working efficiently.